Correlation of cardiovascular and neuroendocrine tests of autonomic function in diabetes

Assessment of cardiac autonomic neuronal function using PET imaging

The autonomic nervous system is the primary extrinsic control of cardiac performance, and altered autonomic activity has been recognized as an important factor in the progression of various cardiac pathologies. Molecular imaging techniques have been developed for global and regional interrogation of pre- and postsynaptic targets of the cardiac autonomic nervous system. Building on established work with the guanethidine analogue ¹²³I-metaiodobenzylguanidine (MIBG) for single-photon emission tomography (SPECT), development of radiotracers and protocols for positron emission tomography (PET) investigation of autonomic signaling has expanded. PET is limited in availability and requires specialized centers for radiosynthesis and interpretation, but the higher resolution allows for improved regional analysis and kinetic modeling provides more true quantification than is possible with SPECT. A wider array of radiolabeled catecholamines, analogues of catecholamines, and receptor ligands have been characterized and evaluated. Sympathetic neuronal PET tracers have shown promise in the identification of several cardiac pathologies. In particular, recent studies have elucidated a mechanistic role for heterogeneous sympathetic innervation in the development of lethal ventricular arrhythmias. Evaluation of cardiomyocyte adrenergic receptor expression and the parasympathetic nervous system has been slower to develop, with clinical studies beginning to emerge. This review summarizes the clinical and the experimental PET tracers currently available for autonomic imaging and discusses their application in health and cardiovascular disease, with particular emphasis on the major findings of the last decade.

Is pancreatic polypeptide response to food ingestion a reliable index of vagal function in type 1 diabetes?

OBJECTIVE

The diagnosis of autonomic neuropathy in diabetic patients is based on cardiovascular reflex tests. Since cardiac function may be affected by arteriosclerosis and cardiomyopathy in type 1 diabetes mellitus, alternative tests reflecting vagal nerve function, in other organ systems, are needed. In this study the pancreatic polypeptide (PP) response to a mixed meal was evaluated in healthy subjects and in recently diagnosed type 1 diabetic patients.

MATERIAL AND METHODS

The PP response was studied at different levels of the vagally mediated reflex arch by application of different stimuli: meal ingestion, i.v. edrophonium (a cholinesterase inhibitor) injection and arginine infusion.

RESULTS

Meal ingestion (stimulation of cerebral/vagal level) resulted in a significant and similar PP response in the two groups; i.v. edrophonium injection (stimulating at the second neuron level) resulted in a smaller increase in PP concentrations in the type 1 diabetic patients as compared with the healthy subjects, whereas direct PP-cell stimulation by arginine infusion resulted in similar increments in PP concentrations in the two groups. Thus, in recently diagnosed type 1 diabetic patients with no known manifestations of diabetic neuropathy, the cholinergic second neuron function of the vagal arch to the pancreas is impaired, whereas intrinsic PP-cell function is unaffected.

CONCLUSIONS

This abnormality in cholinergic second neuron function of the vagal reflex arch and the fact that three of the healthy subjects had no increase in PP concentrations at all during the meal test indicates that PP response to a mixed meal is unsuitable for the diagnosis of autonomic neuropathy in type 1 diabetes. The nature of the defect in the second neuron of the vagal innervation of the pancreas in type 1 diabetes remains to be elucidated.

Quantitative studies of autonomic function

Dysfunction of the peripheral and central autonomic nervous system is common in many neurological and general medical diseases. The quantitative assessment of sympathetic and parasympathetic function is essential to confirm the diagnosis of autonomic failure, to provide the basis for follow-up examinations, and potentially to monitor successful treatment. Various procedures have been described as useful tools to quantify autonomic dysfunction. The most important tests evaluate cardiovascular and sudomotor autonomic function. In this review, we therefore focus on standard tests of cardiovascular and sudomotor function such as heart-rate variability at rest and during deep breathing, active standing, and the Valsalva maneuver, and on the sympathetic skin response. These tests are widely used for routine clinical evaluation in patients with peripheral neuropathies. Refined methods of studying heart-rate variability, baroreflex testing, and detailed measures of sweat output are mostly used for research purposes. In this context, we describe the spectral analysis of slow modulation of heart rate or blood pressure, reflecting sympathetic and parasympathetic influences, and consider various approaches to baroreflex testing, the thermoregulatory sweat test, and the quantitative sudomotor axon reflex test. Finally, we discuss microneurography as a technique of direct recording of muscle sympathetic nerve activity.

Small fiber neuropathy: a common and important clinical disorder

Small fiber neuropathy (SFN) is a neuropathy selectively involving small diameter myelinated and unmyelinated nerve fibers. Interest in this disorder has considerably increased during the past few years. It is often idiopathic and typically presents with peripheral pain and/or symptoms of autonomic dysfunction. Diagnosis is made on the basis of the clinical features, normal nerve conduction studies (NCS) and abnormal specialized tests of small nerve fibers. Among others, these tests include assessment of epidermal nerve fiber density, temperature sensation tests for sensory fibers and sudomotor and cardiovagal testing (QSART) for autonomic fibers. Unless an underlying disease is identified, treatment is usually symptomatic and directed towards alleviation of neuropathic pain.

Diabetic neuropathy in children and adolescents

Diabetic neuropathy (DN) represents a major complication of type 1 diabetes mellitus (T1DM) but there is considerable uncertainty as to its incidence, prevalence, diagnosis and prognosis in pediatric population. Generally, DN is classified as polyneuropathy, focal neuropathy and autonomic neuropathy. The latter seems to be detectable even in asymptomatic children and adolescents with diabetes and is associated with the most serious consequences, such as hypoglycemia unawareness and cardiovascular dysfunction. A near-normal control of blood glucose in the early years after onset of diabetes may delay the development of clinically significant nerve impairment and, therefore, children and adolescents with diabetes represent a critical target for primary prevention of this complication. The aim of this review is to focus on the main clinical, epidemiological and prognostic aspects of DN in children and adolescents with T1DM. Etiopathogenetic theories and diagnostic tools are also reviewed from in a pediatric perspective.

Role of autonomic dysfunction in patients with functional dyspepsia

BACKGROUND

The role of autonomic dysfunction in patients with functional dyspepsia is not completely understood.

AIMS

1. to prospectively assess abnormalities of autonomic function in patients with functional dyspepsia, 2. to assess whether autonomic dysfunction in these patients is associated with a. visceral hypersensitivity or b. delayed gastric emptying or c. severity of dyspeptic symptoms.

PATIENTS

A series of 28 patients with functional dyspepsia and 14 healthy volunteers without gastrointestinal symptoms were studied.

METHODS

All patients and controls were submitted to a battery of five standard cardiovascular autonomic reflex tests, dyspeptic questionnaire, gastric barostat tests and gastric emptying tests.

RESULTS

1. Autonomic function tests showed that both sympathetic and parasympathetic scores of dyspeptic patients were significantly higher than in controls; 2. visceral hypersensitivity was confirmed in dyspeptics in response to proximal gastric distension, demonstrating lower pain threshold; 3. delayed gastric emptying occurred more frequently in patients with functional dyspepsia than in controls; 4. epigastric pain and epigastric burning were significantly more prevalent in patients with definite evidence of autonomic dysfunction; 5. No significant association was found between presence of autonomic dysfunction and presence of visceral hypersensitivity or presence of delayed gastric emptying in patients with functional dyspepsia.

CONCLUSIONS

We concluded that a possible role of autonomic dysfunction in eliciting dyspeptic symptoms could not be determined from alterations in visceral hypersensitivity or delayed gastric emptying. Autonomic dysfunction might not be the major explanation for symptoms associated with functional dyspepsia.

Pulse oximetry for the assessment of autonomic neuropathy in diabetic patients

Altered vascular responses to various thermal stimuli correlate well with the changes of autonomic neuropathy. These responses were assessed by the use of pulse oximetry. Standard cardiac autonomic function tests were performed in normal subjects (n = 12), diabetic patients without autonomic neuropathy (n = 8), and diabetic patients with autonomic neuropathy (n = 7). Autonomic functions in the same patients then were assessed by estimating the severity of vasospasm in response to cold stimulus with the help of pulse oximetry. Percentage fall in oxygen saturation at 15, 30, 60, 90, and 120 sec of exposure to cold stimulus was recorded on pulse oximeter. Time required for recovery and presence or absence of rebound rise following removal of cold stimulus were noted. In diabetics with autonomic neuropathy, the rate of fall in percentage oxygen saturation was significantly slower (p < 0.0001), less intense (p < 0.0001) and with delayed subsequent recovery (p = 0.013), compared to normal subjects. Rebound rise in oxygen saturation was absent in all the diabetics with autonomic neuropathy, compared to 2 of 12 normal subjects (p < 0.0001). We conclude that pulse oximetry may be a potentially useful, simple, and noninvasive bedside method for assessment of diabetic autonomic neuropathy.

Cephalic phase of pancreatic polypeptide release: a valid test of autonomic neuropathy in diabetics?

We investigated pancreatic polypeptide (PP) release by chew and spit sham feeding as a test of autonomic function in 33 patients with diabetes mellitus (DM) and 24 age-matched healthy controls. Plasma samples were taken at nine 5-min time intervals, and PP plasma levels were determined by radioimmunoassay. Autonomic neuropathy (AN) was diagnosed and staged according to four standardized tests of cardiovascular autonomic function. The integrated PP response during sham feeding was decreased in DM patients (1067 + or - 397 vs. 2670 + or - 394 pg/ml/30 min; P < 0.05). The maximum increase of PP plasma levels was significantly different between the DM subgroups with AN (36 + or - 19 pg/ml; n = 13) and without AN (132 + or - 36 pg/ml; n = 20) but varied widely in healthy controls (219 + or - 29 pg/ml; range 20-460 pg/ml). A cutoff of maximum PP increase < 20 pg/ml (specificity 100%) resulted in only 46% sensitivity, while a cutoff < 160 pg/ml (sensitivity 100%) yielded 38% specificity for diagnosis of AN. The diagnostic value of the PP test for identifying individual patients against the variable reference range is therefore limited. Maximum PP increase < 20 pg/ml indicates AN, while maximum PP increase > 160 pg/ml excludes AN in diabetic patients.

Measurement of heart rate variations: influencing factors, normal values and diagnostic impact on diabetic autonomic neuropathy

Different methods of heart rate variation (HRV) measurement are recommended to assess disturbances of cardiovascular reflexes in autonomic polyneuropathies. Influencing factors on the E/I-ratio, HRV variations during Valsalva's maneuver, and change of position from lying down to standing up, as well as age, gender, time of day and satiety were investigated in healthy volunteers. Normal values were then calculated and a total of 100 diabetics with clinically manifest neuropathy of different stages (N0 = 8, N1 = 15, N2a = 24, N2b = 28, N3 = 25), 26 of whom were insulin-dependent and 74 non-insulin-dependent, were examined. Apart from clinical examination, nerve conduction velocity (NCV) measurement of five nerves as well as amplitude measurement of evoked sensory and motor action potentials was performed. No significant influence of the time of day the test was performed could be found. Test reproducibility was good, day-to-day differences did not reach a significant level. This was also true for the test results before and after food intake. Gender-correlated test result differences could not be proven. Statistical analysis showed significant differences between the overall group of diabetics and controls. The difference was most accentuated in the E/I ratio. The further developed the clinical picture of neuropathy, the more pathological were the results obtained in different tests. The results suggest that most changes leading to pathological values of NCV and HRV occur in a clinical stage in which no, or only very slight, clinical signs give evidence of diabetic neuropathy (N0-N1). Therefore, these examinations should, especially, be performed in diabetics with no, or only slight, clinical signs of neuropathy in order to reveal those patients with neuropathic disturbances at an initial stage.

Asymptomatic peripheral nerve dysfunction and vascular reactivity in IDDM patients with and without microalbuminuria

Abnormal vascular reactivity has been implicated in the aetiology of diabetic microvascular disease and we have previously demonstrated enhanced contractility of hand veins to noradrenaline in insulin-dependent diabetic (IDDM) patients with microalbuminuria. We have now assessed the possible contribution of subclinical peripheral nerve dysfunction to exaggerated vascular reactivity in micro-albuminuric patients. Twenty-five IDDM patients (15 with microalbuminuria), none of whom had symptomatic neuropathy, and 10 control subjects were studied. Vasoconstrictor responses were measured in dorsal hand veins using noradrenaline and phenylephrine. Conduction in median, peroneal and sural nerves was assessed using electrophysiology, and autonomic function using standard cardiovascular reflex tests. The noradrenaline dose causing 50% vasoconstriction was significantly lower in the microalbuminuric diabetic subjects compared with normoalbuminuric (3.6(1.7) mean (SEM) ng/min vs 20.1(6.0) ng/min, p = 0.0002) and non-diabetic subjects (35.1(5.0) ng/min; p < 0.0001). However, reactivity to phenylephrine did not differ between the groups. Median nerve motor conduction velocity was significantly slower in microalbuminuric (48.4(1.4) m/s) than in normoalbuminuric (52.7(1.2) m/s, p = 0.04) and non-diabetic subjects (56.7(0.9) m/s, p = 0.0001). In the diabetic group overall, there was a strongly positive linear correlation between vascular response to noradrenaline and conduction velocity in both the median nerve (r = 0.62, p = 0.0009) and peroneal nerve (r = 0.53, p = 0.006). There was no correlation between phenylephrine-induced responses and motor conduction velocity in either nerve, nor were indices of autonomic function correlated with vascular reactivity to either agent.(ABSTRACT TRUNCATED AT 250 WORDS)

Pancreatic polypeptide secretion after insulin infusion and protein meal in juvenile type 1 diabetic subjects

An impaired pancreatic polypeptide response (PP) after hypoglycemia has been described in type I diabetic patients with overt autonomic neuropathy. Some authors have suggested that PP release might be useful as sensitive indicator of autonomic neuropathy. The meal test is safer and simpler than the insulin infusion test as PP stimulus. The aim of this study was to compare PP response to insulin infusion and protein meal test and to correlate these responses to the presence of measurable neuropathic disturbances. We thus studied 13 IDDM children and adolescents and 6 normal children. In diabetics the PP response to both tests was not different from that of the control subjects, but PP response to insulin infusion was inversely correlated to the duration of illness and was significantly lower in subjects with pathological heart rate response when compared to the control group. PP responses to the two stimuli were not correlated. We suggest that reduced PP response to hypoglycemia is an early sign of autonomic neuropathy as well as impairment of beat-to-beat variation when impaired PP response to meal test is still not evident.

Noradrenaline response to edrophonium in primary autonomic failure: distinction between central and peripheral damage

The effects of intravenous edrophonium on plasma noradrenaline were studied in 12 subjects with primary autonomic failure. 5 had clinical features of central autonomic damage, 4 with parkinsonism (PD) and 1 with multiple system atrophy (MSA); the other 7 had clinical features of progressive autonomic failure without detectable central neurological damage (PAF). After edrophonium all but 1 subject showed falls in heart rate of about 6 beats/min; there were no consistent blood pressure changes. The 5 PD/MSA patients all had normal plasma noradrenaline responses to edrophonium, with rises of 35-66% within 2-8 min of injection. 6 of the 7 PAF patients had no response or very small responses (-6 to +11%), while the other subject had a normal response (70% rise). The noradrenaline response to edrophonium may provide a simple way to differentiate between central and peripheral autonomic damage.

Clinical aspects of diabetic neuropathies

Diabetic neuropathy is a common complication of diabetes that may be associated both with considerable morbidity (painful polyneuropathy, neuropathic ulceration) and mortality (autonomic neuropathy). The epidemiology and natural history of diabetic neuropathy is clouded with uncertainty, largely due to confusion in the definition and measurement of this disorder. We have reviewed a variety of the clinical manifestations associated with somatic and autonomic neuropathy and discussed current views related to the management of the different abnormalities. Although unproven, the best evidence suggests that near normal control of blood glucose in the early years following onset of diabetes may help delay the development of clinically significant nerve impairment. Intensive therapy to achieve normalization of blood glucose may also lead to reversibility of early diabetic neuropathy, but again this is unproven. Our ability to manage successfully the many different manifestations of diabetic neuropathy depends ultimately on our success in uncovering the pathogenic processes underlying this disorder. The recent resurgence of interest in the vascular hypothesis, for example, has opened up new avenues of investigation for therapeutic intervention. Paralleling our increased understanding of the pathogenesis of diabetic neuropathy, there must be refinements in our ability to measure quantitatively the different types of defects that occur in this disorder. These tests must be validated and standardized to allow comparability between studies and more meaningful interpretation of study results.

Autonomic neuropathy: its diagnosis and prognosis

Autonomic neuropathy is now well established as a relatively common and significant complication of diabetes mellitus. Its importance has been clarified in recent years during which the extent of autonomic control over all areas of body function has been defined. Using simple cardiovascular reflex tests, autonomic abnormalities can be demonstrated without any corresponding symptoms. The often stated concept of 'patchy' involvement in diabetic autonomic neuropathy should now be rejected as too should the view that autonomic neuropathy is either 'present' or 'absent' based on a single test result. When generalized and predominantly metabolic disturbances, as in diabetes, give rise to impaired nerve function, autonomic as well as somatic components of the nerve are affected. Where damage is severe this leads to the characteristic florid picture of symptomatic autonomic neuropathy with its particularly poor prognosis. For the physician in a busy clinic, much of the theoretical and experimental basis for autonomic neuropathy may not appear of direct relevance. However, he has now to be aware of the clinical implications of autonomic damage in the diabetic. This may have particular relevance in the care of the diabetic foot (see Chapter 10), the recognition of many of the vague symptoms associated with autonomic damage, the treatment of disabling features such as postural dizziness and nocturnal diarrhoea, and an awareness of the poor prognosis associated with symptomatic autonomic neuropathy. He will also need to be alert to the dangers of general anaesthesia in such patients, and the possibility of sudden unexpected deaths. Diabetic autonomic neuropathy causes widespread abnormalities, some of which are clinically apparent, some of which can be detected by sensitive tests, and others which have yet to be discovered. Inclusion of the neuropeptides and other hormones within the compass of autonomic control has opened up a whole new area of investigative interest, with many complex interrelationships which still need to be unravelled. This should lead to better understanding of the pathophysiological processes that cause damage to diabetic nerves. With so much research effort directed towards better glycaemic control and aldose reductase inhibitors (see Chapter 8), it may eventually be possible to reverse or prevent this potentially disabling and lethal complication of diabetes.